Optical absorption techniques such as non-dispersive infrared (NDIR) measurement have been recognized for many years as sensitive, stable and reliable methods of gas concentration measurement. In a typical NDIR method, the selective absorption of infrared radiation by certain gas species of interest is measured to determine the concentration of the target gas in a sample. This has a wide variety of applications—for example, NDIR measurements detecting absorption of radiation by carbon dioxide and other gases, such as carbon monoxide or hydrocarbons, are commonly used to monitor atmospheric composition or automotive exhaust, as well as in fire detectors.
A conventional NDIR instrument typically comprises the following elements:
a source of radiation (usually infrared), such as an incandescent lamp or another electrically heated element that serves as a blackbody emitter, e.g. a silicon carbide rod or nichrome filament;
a narrow bandwidth interference filter arranged to ensure that only radiation absorbed by gas of interest is measured;
a gas chamber for containing a sample including the target gas of interest; and
a photodetector for detecting radiation transmitted by the sample and transforming the intensity of the detected radiation into an electrical signal.
Often, the intensity of incident radiation may be modulated by a mechanical chopper or by an electrically modulated radiation source (“on and off” regime).
Such a device is termed a “one channel” NDIR sensor and represents the most basic NDIR device. This type of instrument is relatively inexpensive, but does not provide any kind of compensation for instrument drift over time which may occur due to the radiation source and/or the photodetector ageing, or accumulation of dirt and dust in the optical path, for example. As a result, “one channel” NDIR instruments need to be calibrated relatively often.
“Two channel” NDIR sensors have a signal channel and a reference channel. The signal channel operates in exactly the same way as the “one channel” device describes above, with the wavelength of the band pass filter adjusted to the absorption wavelength(s) of the gas of interest.
The reference channel usually works in another wavelength band, at which the target gas species does not absorb. This provides a base line for the signal channel. The differential signal between the signal and reference channels, normalized on reference channel intensity, gives an absorption signal which is stable with respect to any intensity drift resulting from the radiation source (or detector). In typical “two channel” sensors, the source of radiation has a wide spectral output, comprising both the signal and reference wavelengths. Another type of “two channel” NDIR sensor comprises two photodetectors and includes two separate gas cells into which the emission from the radiation source is split along paths of equal lengths. One cell is filled with non-absorptive (inert) gas to provide a reference channel, and the other with the sample gas (including the gas of interest). Such sensors can work with good stability with the two channels working on the same wavelength (corresponding to an absorption line of the target gas), but the requirement for a separate, sealed gas reference cell containing an inert gas is a serious limitation in a portable, low cost design.
As a result, the use of a reference channel working on a wavelength where the target gas does not absorb is preferred in two channel NDIR instruments and has been found to provide a high degree of compensation of source radiation drift, and thus good measurement accuracy. Practically all such devices use an incandescent bulb as the source of radiation since it provides the necessary wide wavelength radiation band. However, such radiation sources are slow (typically, the response time is more than 100 milliseconds) and has significant power consumption (200 milliwatts or more). As such, these components are not suitable for portable, low power sensors which can typically support a power consumption of no more than 1-2 milliwatts.
Radiati on sources such as light emitting diodes (LEDs), instead, are very fast (the response time is of the order of a few microseconds) and can be used in regimes having a power consumption of less than one milliwatt. However, LEDs suffer from the problem that their output power and emitted radiation wavelength depend significantly on temperature. These temperature dependences have a fundamental nature and can not be avoided in the design of the LED.
Another problem encountered in the use of LEDs, as compared with bulbs, is the relatively narrow wavelength range of emission (usually not exceeding one micrometer). As a result, an LED cannot be used the same way as a bulb in a conventional NDIR sensor, since it cannot provide emission on a reference wavelength (in addition to the absorption wavelength). Using a second LED to provide the reference wavelength does not assist, because for LEDs working on different wavelengths, the temperature dependences of parameters (intensity and wavelength of emission) are different and cannot be precisely compensated.
In accordance with the present invention, an optical absorption gas analyser for determining the concentration of a target gas in a sample, comprises:
a chamber for containing the sample in use;
a radiation source assembly arranged to emit radiation into the chamber;
a first radiation detector assembly arranged to detect radiation transmitted along a first optical path through the chamber;
a second radiation detector assembly arranged to detect radiation transmitted along a second optical path through the chamber, wherein the length of the second optical path which the sample can intercept is shorter than that of the first optical path; and
a processor adapted to generate a sensing signal SS based on the detected radiation transmitted along the first optical path and a reference signal SR based on the detected radiation transmitted along the second optical path, and to determine the concentration of the target gas in the sample based on a comparison of the sensing signal with the reference signal.
By arranging for radiation to be detected along a second optical path which is shorter than the first, the invention provides a reference channel which operates using the same radiation as the signal channel, yet does not require the provision of a separate (inert) cell, since both paths pass through the same chamber. The relatively short length of the second optical path with which the sample can interact (compared with that of the first optical path) means that absorption in the reference channel is suppressed and can be used to accurately compensate for drift. Preferably, the length of the second optical path with which the sample can interact is made as short as possible, and in any case significantly shorter than that of the first optical path. As a result any losses caused by absorption in the reference path will be small.
This arrangement makes it possible to use an LED or other fast-response radiation source since both the signal and reference channels can operate at the same (or overlapping) waveband.
It is of key importance that, within the chamber, the length of the second optical path which the sample can intercept (i.e. to which the sample has access) is shorter than that of the first optical path, however in certain embodiments it is preferred that the entire length of the first optical path (i.e. from source to detector) is greater than that of the second.
Preferably, the generated sensing signal SS depends upon the concentration of the target gas in the sample and on the intensity of radiation emitted by the radiation source assembly.
Preferably, the generated reference signal SR depends upon the intensity of radiation emitted by the radiation source assembly, and is substantially independent of the concentration of the target gas in the sample. This is achieved by arranging the length of the second optical path with which the sample can interact to be as short as possible.
Advantageously, the processor is adapted to determine the concentration of the target gas in the sample by generating a differential absorption signal SA corresponding to the difference between the sensing signal SS and the reference signal SR:SA=SS−SR In this way, the output signal takes account of any variation in the radiation emitted by the source.
Preferably, the processor is further adapted to generate a normalised differential absorption signal SN relative to the reference signal SR:SN=SA/SR This provides a dimensionless output which is dependent on target gas concentration but not the intensity of the source radiation and is also independent of perturbations in the emitted radiation wavelengths caused by temperature variations.
The source and detectors could be arranged at various opposing sides of the chamber, making use of different dimensions of the chamber to define the first and second optical paths. However, in preferred embodiments, at least one optical guiding assembly is disposed within the chamber to define at least one of the first and second optical paths. This can be used to maximise the length of the first optical path, and minimise that of the second.
Any suitable optical elements could be used to construct the optical guiding assembly, including, if desired, parts of the chamber wall itself. Advantageously, the optical guiding assembly comprises a partially-reflective element arranged to split the first optical path from the second, preferably a partially-reflective mirror or a partially-reflective prism. Such an element could, for example, make use of a semi-silvered mirror. However, preferably, the partially-reflective element comprises a mirror surface having one or more non-reflective regions through which the first or second optical path passes.
In a particularly preferred embodiment, the optical guiding assembly comprises a first mirror having a transmissive portion, the first mirror being arranged to receive radiation emitted by the radiation source assembly and to transmit a portion of the radiation through the transmissive portion to the second detector assembly, and a second mirror arranged to receive radiation reflected by the first mirror and reflect it towards the first detector assembly through the transmissive portion of the first mirror. This has been found to provide a particularly compact means of achieving a long first path and short second path within the chamber. Preferably, the first and/or second mirrors have a shape arranged to focus incident radiation, preferably spherical or parabolic.
In other preferred examples, the optical guiding assembly comprises a reflective element arranged to intercept a portion of the radiation emitted by the radiation source assembly, preferably a mirror, a reflective portion of the chamber's interior or an optical fibre. In such cases, only part of the radiation emitted by the source is incident on the reflective element, the remainder continuing past the element on the other of the two optical paths.
The optical guiding assembly inside the sample chamber may comprise solely reflective surfaces, such as mirrors, in which case the whole of the first and/or second optical path can be intercepted by the sample in the chamber. In other cases, one or more optical components may take the form of radiation-transparent elements (e.g. prisms or optical fibres), within which the optical path cannot interact with the sample. In one preferred embodiment, the second optical path is substantially wholly contained within one or more optical elements making up the optical guiding assembly. In this way, absorptions on the reference channel can be eliminated entirely.
As indicated above, preferably the first optical path should be significantly longer than the second. Advantageously, the length of the first optical path which the sample can intercept is at least 3 times that of the second optical path, preferably more than 5 times the length and still preferably more than 8 times the length.
Preferably, the length of the second optical path which the sample can intercept is less than or equal to 40 mm, preferably less than or equal to 20 mm, still preferably less than or equal to 10 mm.
The arrangement is particularly advantageous when implemented using a radiation source assembly comprising a narrow band emitter adapted to emit radiation over a waveband corresponding to an absorption wavelength of the target gas. Preferably, the width of the waveband emitted by the radiation source assembly is less than or equal to 1 micron. In certain cases, the radiation source assembly may additionally comprise a filter for controlling the waveband emitted. Advantageously, the filter is an interference filter.
It is especially preferred that the radiation source assembly comprises an emitter having a response time of less than or equal to 100 milliseconds, preferably less than 1 milliseconds, still preferably less than 50 microseconds.
In preferred embodiments, the radiation source assembly comprises a LED. Advantageously, the radiation is infrared radiation.
Preferably, the first and second detector assemblies are adapted to detect radiation of the same wavelength(s) as each other, corresponding to an absorption wavelength of the target gas. This ensures that the compensation made using the reference channel is most accurate. In certain embodiments, the first and second detector assemblies each comprise a filter for controlling the wavelength(s) of radiation detected. Advantageously, the first and second detector assemblies each comprise a photodetector, preferably a photodiode, a pyroelectric photodetector or a thermocouple photodetector.
Preferably, the optical absorption gas analyser further comprises a controller adapted to control power supplied to the radiation source assembly. In particularly preferred embodiments, the controller is adapted to perform pulse width modulation control, supplying the radiation source assembly with discrete pulses of power. This enables the source to be controlled in particularly low power consumption regimes. Advantageously, the pulses have a duration of between 15 microseconds and 100 milliseconds, preferably approximately 20 microseconds. The pulse width modulated signal may have a period of between 100 microseconds and 10 seconds. The pulse width modulated signal advantageously has a duty cycle of between 0.01% and 50%, preferably approximately 0.04% to 0.8%.
The analyser could be connected to an external power source in order to supply power to the radiation source and processing components. However, it is preferred that that the analyser further comprises a power source so that the device is fully portable. Preferably the power source comprises a battery, solar cell or solar-powered battery. In particularly preferred embodiments, a highly efficient solar battery (up to 28% efficient) is provided, which can deliver several milliwatts of energy even in low illumination, significantly economising on battery power.
As noted previously, certain radiation source types have a significant dependence on temperature. The same is true for certain types of radiation detector, in particular semiconductor photodetectors. Therefore, preferably, the first and second detector assemblies are located adjacent one another, preferably arranged on the same or adjacent faces of the chamber. This keeps temperature discrepancies due to position of the elements to a minimum.
To alleviate this problem still further, the first and second detector assemblies are preferably arranged in thermal contact with one another. This may be achieved, for example, by mounting both detectors on a thermally conductive plate. In other embodiments, a thermal conductor may be connected between the first and second detector assemblies.
Preferably, the chamber is provided with at least one aperture for gas ingress from the surrounding atmosphere. This enables an atmosphere to be monitored in real time. However, in alternative situations, the sample could be input to the chamber by an operator for evaluation.
A further problem that may be encountered is high humidity environments which can lead to water condensation inside the chamber. If this occurs on optical surfaces, the water droplets will cause absorption of radiation as well as scattering, distorting the measurements obtained from the instrument. Therefore, preferably, the optical absorption gas analyser further comprises a condensation preventor for preventing the condensation of water on optical surfaces forming part of the first and/or second optical paths. This could take active or passive forms. In one embodiment, the condensation preventor comprises a thermal isolator arranged to thermally isolate each optical surface. Preferably, the thermal isolator forms a substrate upon which the optical surface is supported, preferably a mirror surface. Advantageously, the thermal isolator is formed of a material with low thermal capacity and low thermal conductivity, preferably a polymer, still preferably a polymer foam. Examples of suitable materials include (expanded) polyisocyanurate, eg Trymer™ 1800, and Primaplex™.
In other examples, the condensation preventor preferably comprises a heater arranged to heat each optical surface. Preferably, the optical surface comprises a conductive material and is provided with connectors for connection to a power supply to thereby form a heating element. The optical absorption gas analyser preferably further comprises a controller for controlling power supply to the heating element. Advantageously, the optical surface is reflective and comprises a metallic material, preferably aluminium, silver or gold.